Iii-nitride transistor with enhanced doping in base layer

ABSTRACT

A vertical trench MOSFET comprising: a N-doped substrate of a III-N material; and an epitaxial layer of the III-N material grown on a top surface of the substrate, a N-doped drift region being formed in said epitaxial layer; a P-doped base layer of said III-N material, formed on top of at least a portion of the drift region; a N-doped source region of said III-N material; formed on at least a portion of the base layer; and a gate trench having at least one vertical wall extending along at least a portion of the source region and at least a portion of the base layer; wherein at least a portion of the P-doped base layer along the gate trench is a layer of said P-doped III-N material that additionally comprises a percentage of aluminum.

FIELD OF THE INVENTION

The present invention relates generally to III-Nitride transistors, andin particular to a GaN based vertical trench MOSFETs.

BACKGROUND

GaN-based transistors are increasingly used in power devices.AIGaN/GaN-based lateral field-effect transistors, wherein polarizationcharges at the heterointerface produce a high-density, high-mobilitytwo-dimensional electron gas (2DEG) and thus effectively reduce theon-state resistance, are predominantly used. Lateral GaN transistors canbe fabricated on low-cost, large-diameter Si substrates. However, thethreshold voltage of most lateral GaN transistors is not high enough foruse in high-power applications such as automotive applications, where athreshold voltage above 3-5 V is preferred in order to prevent falseoperation caused by noise. Furthermore, in these transistors increasingthe breakdown voltage is achieved by increasing the gate-drain spacing,which reduces the effective current density and increases the chip sizeand cost for a required amperage rating.

Alternatively, vertical GaN devices on free-standing GaN substrates havebeen attracting attention. In vertical devices the breakdown voltage isincreased by increasing the thickness of the drift region withoutsacrificing the device size, so that high-power density chips can berealized. The paper “Vertical GaN-based trench metal oxide semiconductorfield-effect transistors on a free-standing GaN substrate with blockingvoltage of 1.6 kV”, by Tohru Oka, Yukihisa Ueno, Tsutomu Ina, and KazuyaHasegawa (Applied Physics Express 7, 021002 (2014)), discloses verticalGaN-based trench metal-oxide-semiconductor field-effect transistors on afree-standing GaN substrate having a blocking voltage of 1.6 kV.

FIG. 1 illustrates schematically a prior art Vertical GaN-based trenchMOSFET 10 on a free-standing GaN substrate 12. Substrate 12 is astrongly N doped GaN substrate 12, on which is formed a GaN epitaxiallayer 14. A lightly N-doped drift region 16 is formed in the bottom oflayer 14, on top of substrate 12. According to the present disclosure,lightly doped can mean having a doping lower than 1E18 cm⁻³. A stronglyP-doped base layer 18 is formed in layer 14 on top of drift region 16,and a strongly N-doped source region 20 is formed on top of base layer18. According to the present disclosure, strongly doped can mean havinga doping higher than 1E18 cm⁻³. A source contact 22 can be formed onsource region 20. A gate trench 24 having at least one vertical wall 26extending along a portion of source region 20 and a portion of baselayer 18, has a bottom wall 28 in contact with the drift region 16. Aninsulating layer 30 covers the inside of trench 24. A gate region 32 canbe formed on top of insulating layer 30. A gate contact 34 can be formedon gate region 32. A drain contact 35 can be formed on the bottom ofsubstrate 12.

The P-doping of base layer 18 can be accomplished by incorporating aP-dopant, such as magnesium, in epitaxial layer 14. It has been notedhowever that P-type doping in GaN is usually inefficient. This isbecause: 1. magnesium dopants in GaN are largely passivated by hydrogenatoms; 2. magnesium doping has a high ionization energy. InsufficientP-type doping in GaN leads to reduced performances of the transistor 10,such as low threshold voltage and high base resistance.

SUMMARY

The present disclosure relates to a vertical III-N trench MOSFET, suchas a vertical GaN trench MOSFET, wherein at least a portion of theP-doped base layer along the gate trench comprises a percentage ofaluminum, thus forming an heterostructure with regions below and/orabove.

An embodiment of the present disclosure relates to a vertical trenchMOSFET comprising a N-doped substrate of a III-N material; an epitaxiallayer of the III-N material grown on a top surface of the substrate, aN-doped drift region being formed in said epitaxial layer; a P-dopedbase layer of said III-N material, the base layer being formed on top ofat least a portion of the drift region; a N-doped source region of saidIII-N material; the source region being formed on at least a portion ofthe base layer; and a gate trench having at least one vertical wallextending along at least a portion of the source region and at least aportion of the base layer; wherein at least a portion of the P-dopedbase layer along the gate trench is a layer of said P-doped III-Nmaterial that additionally comprises a percentage of aluminum.

According to an embodiment of the present disclosure, said III-Nmaterial is GaN.

According to an embodiment of the present disclosure, the percentage ofaluminum in the layer of said P-doped III-N material varies vertically,(or along a direction normal to the plane of the substrate, as opposedto the plane of the surface that represents a horizontal plane).

According to an embodiment of the present disclosure, the percentage ofaluminum is lower than 20%.

According to an embodiment of the present disclosure, the layer of saidP-doped III-N material that additionally comprises a percentage ofaluminum is an AlGaN layer grown on a Ga face of an underlying GaNlayer; wherein the percentage of aluminum of the AlGaN layer decreasesfrom bottom to top (where e.g. epitaxial growth normal to the surface ofthe substrate grows from bottom to top).

According to an embodiment of the present disclosure, the percentage ofaluminum decreases from 20 to 0% from bottom to top.

According to an embodiment of the present disclosure, said layer of saidP-doped III-N material that additionally comprises a percentage ofaluminum is an AlGaN layer grown on a N face of an underlying GaN layer;wherein the percentage of aluminum of the AlGaN layer increases frombottom to top.

According to an embodiment of the present disclosure, the percentage ofaluminum increases from 0 to 20% from bottom to top.

According to an embodiment of the present disclosure, said layer of theP-doped base layer that additionally comprises a percentage of aluminumis grown on a P-doped base layer formed in the epitaxial layer on top ofthe drift region.

The present disclosure, also relates to a method of fabricating verticaltrench MOSFET comprising: providing a substrate of strongly N-dopedIII-N material; forming on the substrate an epitaxial layer of the III-Nmaterial; forming in the epitaxial layer a lightly N-doped drift regionof a III-N material, in contact with the substrate; forming on theepitaxial layer a strongly P-doped region of said III-N materialcomprising a percentage of aluminum; forming on the base layer astrongly N-doped source region of said III-N material; and forming agate trench having at least one vertical wall extending along at least aportion of the source region and at least a portion of the base layer.

According to an embodiment of the present disclosure, said III-Nmaterial is GaN.

According to an embodiment of the present disclosure, the percentage ofaluminum varies vertically.

According to an embodiment of the present disclosure, said forming onthe epitaxial layer a strongly P-doped region of said III-N materialcomprising a percentage of aluminum comprises growing an AlGaN layer ona Ga face of the GaN Epitaxial layer; wherein the percentage of aluminumof the AlGaN layer decreases from bottom to top.

According to an embodiment of the present disclosure, said underlyingGaN layer is a strongly P-doped region formed in said epitaxial layerabove the lightly N-doped drift region.

According to an embodiment of the present disclosure, the percentage ofaluminum decreases from 20 to 0%.

According to an embodiment of the present disclosure, said forming onthe epitaxial layer a strongly P-doped region of said III-N materialcomprising a percentage of aluminum comprises growing an AlGaN layer onan N face of the GaN Epitaxial layer; wherein the percentage of aluminumof the AlGaN layer increases from bottom to top.

According to an embodiment of the present disclosure, the percentage ofaluminum increases from 0 to 20%.

According to an embodiment of the present disclosure, said forming onthe epitaxial layer a strongly P-doped region of said III-N materialcomprising a percentage of aluminum comprises growing said regioncomprising a percentage of aluminum on a P-doped base layer formed inthe epitaxial layer on top of the drift region.

The present disclosure also relates to a semiconductor circuitcomprising: a first GaN layer; and a second GaN layer grown on the firstlayer, wherein the second layer comprises a percentage of aluminum thatvaries with the distance to the first layer.

According to an embodiment of the present disclosure, the second layeris grown on a Ga face of the first layer and the percentage of aluminumdecreases when the distance to the first layer increases; or the secondlayer is grown on a N face of the first layer and the percentage ofaluminum increases when the distance to the first layer increases

BRIEF DESCRIPTION OF THE DRAWINGS

The invention(s) may be better understood by referring to the followingfigures. The components in the figures are not necessarily to scale,emphasis instead being placed upon illustrating the principles of theinvention. In the figures, like reference numerals designatecorresponding parts throughout the different views.

FIG. 1 illustrates a cross-section of a portion of a prior art verticalGaN trench MOSFET.

FIG. 2 illustrates a cross-section of a vertical trench MOSFET accordingto an embodiment of the present disclosure.

FIG. 3 illustrates a cross-section of a vertical trench MOSFET accordingto an embodiment of the present disclosure.

DETAILED DESCRIPTION

FIG. 2 illustrates schematically a Vertical GaN-based trench MOSFET 30according to an embodiment of the present disclosure. MOSFET 30comprises a free-standing strongly N doped GaN substrate 32, on which isformed a GaN epitaxial layer 34. A lightly N-doped drift region 36 isformed in the bottom of layer 34, on top of substrate 32. A stronglyP-doped base layer 38 is formed in layer 34 on top of drift region 36.According to an embodiment of the present disclosure, the top surface ofbase layer 38 is a Ga face, or wurtzite Ga face, of the GaN crystalstructure, and a layer 40 of P-doped GaN material that additionallycomprises a percentage of aluminum was grown on top of base layer 38.According to an embodiment of the present disclosure, substrate 32 isarranged such that the [0001] direction is normal to the surface of thesubstrate and outward bound, (as opposed to a direction toward theinside of the substrate, which would be inward bound). By Ga-face it ismeant that Ga is found on the top position of the {0001} bilayer,corresponding to the [0001] polarity, or GaN(0001). By convention, the[0001] direction is given by a vector pointing from a Ga atom to anearest-neighbor N atom. It is important to note that the [0001] and[0001] surfaces of GaN are nonequivalent and differ in their chemicaland physical properties. Ga face epitaxial layers can be manufactured byMOCVD (MetalOrganic Chemical Vapor Deposition), as described for examplein the document “Two-dimensional electron gases induced by spontaneousand piezoelectric”, by O. Ambacher et al.; JOURNAL OF APPLIED PHYSICSVOLUME 85, NUMBER 6; 15 MARCH 1999.

According to an embodiment of the present disclosure, MOSFET 30comprises a layer 40 of AlGaN grown on the Ga face of base layer 38.According to an embodiment of the present disclosure, MOSFET 30 furthercomprises a strongly N-doped GaN source region 42 grown on top of AlGaNlayer 40. Base layer 38 and layer 40 form together a base layer orregion of MOSFET 30.

A source contact 44 can be formed on source region 42. A gate trench 46having at least one vertical wall 48 extends along a portion of sourceregion 42 and a portion of base layer 40, 38, and has a bottom wall 50,preferably in contact with the drift region 36. An insulating layer 52covers the inside of trench 46. A gate region 54 can be formed on top ofinsulating layer 52. A gate contact 56 can be formed on gate region 54.A drain contact 58 can be formed on the bottom of substrate 32.

According to an embodiment of the present disclosure, the percentage ofaluminum of AlGaN layer 40 decreases from bottom (at the interface withbase layer 38) to top (at the interface with source layer 42). Accordingto an embodiment of the present disclosure, the percentage of aluminumin layer 40 can be of 20% at the junction with base layer 38 and 0% atthe junction with source region 42. The document “Two dimensionalelectron gases induced by spontaneous and piezoelectric polarization inundoped and doped AlGaN/GaN heterostructures” by O. Ambacher et al.;JOURNAL OF APPLIED PHYSICS VOLUME 87, NUMBER 11; JANUARY 2000, herebyincorporated by reference, teaches that in a GaN/AlGaN/GaNheterostructure with Ga-face polarity, a 2DEG is formed close to thelower AlGaN/GaN interface due to the piezoelectric and spontaneouspolarization in the heterostructure. The Ambacher document does notteach an AlGaN layer having an aluminum percentage that varies.

The present disclosure provides a GaN/AlGaN/GaN heterostructure such ascomprising regions/layers 42, 40 and 38 with a variable percentage ofaluminum in the AlGaN layer 40, wherein AlGaN layer 40 is formed on a Gaface GaN region 38 and wherein the percentage of aluminum decreases frombottom to top. The decreasing aluminum composition in layer 40 generatesa built-in polarization electric field which assists ionization ofP-type dopants to form higher concentration of holes in the base layer.According to an embodiment of the present disclosure, higher holeconcentration in the base layer enhances the threshold voltage, andreduces the base resistance.

According to an embodiment of the present disclosure, the percentage ofaluminum in region 40 varies continuously. It can vary linearly but canalso vary non-linearly, depending on the desired repartition of holes inregion 40. According to an embodiment of the present disclosure, thepercentage of aluminum in region 40 can vary from 0 at the top to 20% atthe bottom. The percentage of aluminum in region 40 can also vary alongdifferent ranges.

According to an embodiment of the present disclosure, region 40 cancomprise a layer of GaN (layer xx) on top of a layer of AlGaN (layer yy)with a constant aluminum composition in the AlGaN layer. In such a case,the concentration peak of the holes in region 40 will be at theinterface between layer xx and layer yy.

According to an embodiment of the present disclosure, the top surface ofbase layer 38 can alternatively be a N face, or wurtzite N face, of theGaN crystal. In such an embodiment, substrate 32 is arranged such thatthe [0001] direction is normal to the surface of the substrate andoutward bound. N-face epitaxial layers can be manufactured by PIMBE(Plasma Induced Molecular Beam Epitaxy), as described for example in thedocument “Two-dimensional electron gases induced by spontaneous andpiezoelectric”, by O. Ambacher et al.; JOURNAL OF APPLIED PHYSICS VOLUME85, NUMBER 6; 15 MARCH 1999, hereby incorporated by reference.

According to such an embodiment of the present disclosure, thepercentage of aluminum of AlGaN layer 40 increases from bottom to topbecause in pseudomorphic GaN/AlGaN/GaN heterostructures with N-facepolarity, electrons are located close to the upper GaN/AlGaN interface.The percentage of aluminum of AlGaN layer 40 increases from bottom totop so that the built-in polarization electric-field assists generationof holes in layer 40.

According to embodiments of the present disclosure, a P-type dopant canbe magnesium, wherein doping with Mg is conducted by introducingmagnesium precursors during the MOCVD or MBE growth of the (Al)GaNlayer.

FIG. 3 illustrates a vertical MOSFET 30′ according to an embodiment ofthe present disclosure, where region 40 was grown directly on the top ofdrift region 36. MOSFET 30′ is identical in structure to MOSFET 30 asshown in FIG. 2, except that it comprises no epitaxial base layer 38. InMOSFET 30′, the base layer is entirely comprised of region 40. Accordingto an embodiment of the present disclosure, a preferred height of region40 is 200 nm to 2 um. An embodiment as illustrated in FIG. 2, with abase layer 38, can have a thicker base than an embodiment a illustratedin FIG. 3.

It is noted that a HEMT according to embodiments of the presentdisclosure will be suitable for High Voltage GaN device applicationsincluding Electrical Vehicles, Trucks, Traction application, HVtransmission lines and naval applications where high efficient powerswitches are required. The total available market of discrete powerdevices is expected to reach $ 20 Billion by 2020. The HV market inwhich HV GaN HEMT can target is estimated at $ 8 Billion by 2020. Theinsertion of GaN based power devices in the aforementioned applicationsis of significant interest to car manufacturers, as well as energy anddefense industries, due to the superior material properties of GaNHEMTs. Further, GaN based power devices are considered to be the maincandidate to lead future roadmaps of energy efficient products. HEMTsaccording to the present disclosure are particularly useful inapplications that require 1200V blocking capability, for example for theelectrification of next generation vehicles. The global requirement forCO2 emission reduction and the drive in the U.S. to reduce dependence onforeign oil are driving the market pull for energy efficientsemiconductor devices that are superior in performance to the existingSilicon device which will enable operations at higher temperature thatare not addressed by smaller band-gap (Eg=1.1 eV) of silicon based powerdevices.

The foregoing description of the preferred embodiments of the presentinvention has been presented for purposes of illustration anddescription. It is not intended to be exhaustive or to limit theinvention to the precise form or to exemplary embodiments disclosed.Obviously, many modifications and variations will be apparent topractitioners skilled in this art. Similarly, any process stepsdescribed might be interchangeable with other steps in order to achievethe same result. The embodiment was chosen and described in order tobest explain the principles of the invention and its best mode practicalapplication, thereby to enable others skilled in the art to understandthe invention for various embodiments and with various modifications asare suited to the particular use or implementation contemplated.

For example, the present disclosure was made with respect to addingaluminum to the GaN material of a base layer of a vertical trenchtransistor to enhance the P doping of the base layer. However, thepresent disclosure is not limited to the above-disclosed materials orstructures. For example, one can add indium to the GaN material insteadof aluminum, or one can replace GaN by ZnO. The descriptions of thefigures above are to be read by replacing Aluminum with Indium or GaNwith ZnO.

It is intended that the scope of the invention be defined by the claimsappended hereto and their equivalents. Reference to an element in thesingular is not intended to mean “one and only one” unless explicitly sostated, but rather means “one or more.” Moreover, no element, component,nor method step in the present disclosure is intended to be dedicated tothe public regardless of whether the element, component, or method stepis explicitly recited in the following claims. No claim element hereinis to be construed under the provisions of 35 U.S.C. Sec. 112, sixthparagraph, unless the element is expressly recited using the phrase“means for . . . . ”

It should be understood that the figures illustrated in the attachments,which highlight the functionality and advantages of the presentinvention, are presented for example purposes only. The architecture ofthe present invention is sufficiently flexible and configurable, suchthat it may be utilized (and navigated) in ways other than that shown inthe accompanying figures.

Furthermore, the purpose of the foregoing Abstract is to enable the U.S.Patent and Trademark Office and the public generally, and especially thescientists, engineers and practitioners in the art who are not familiarwith patent or legal terms or phraseology, to determine quickly from acursory inspection the nature and essence of the technical disclosure ofthe application. The Abstract is not intended to be limiting as to thescope of the present invention in any way. It is also to be understoodthat the steps and processes recited in the claims need not be performedin the order presented.

Also, it is noted that the embodiments may be described as a processthat is depicted as a flowchart, a flow diagram, a structure diagram, ora block diagram. Although a flowchart may describe the operations as asequential process, many of the operations can be performed in parallelor concurrently. In addition, the order of the operations may bere-arranged. A process is terminated when its operations are completed.A process may correspond to a method, a function, a procedure, asubroutine, a subprogram, etc. When a process corresponds to a function,its termination corresponds to a return of the function to the callingfunction or the main function.

The various features of the invention described herein can beimplemented in different systems without departing from the invention.It should be noted that the foregoing embodiments are merely examplesand are not to be construed as limiting the invention. The descriptionof the embodiments is intended to be illustrative, and not to limit thescope of the claims. As such, the present teachings can be readilyapplied to other types of apparatuses and many alternatives,modifications, and variations will be apparent to those skilled in theart.

1-9. (canceled)
 10. A method of fabricating vertical trench MOSFETcomprising: providing a substrate of strongly N-doped III-N material;forming on the substrate an epitaxial layer of the III-N material;forming in the epitaxial layer a lightly N-doped drift region of a III-Nmaterial, in contact with the substrate; forming on the epitaxial layera strongly P-doped region of said III-N material comprising a percentageof aluminum; forming on the base layer a strongly N-doped source regionof said III-N material; and forming a gate trench having at least onevertical wall extending along at least a portion of the source regionand at least a portion of the base layer.
 11. The method of claim 10,wherein said III-N material is GaN.
 12. The method of claim 10, whereinthe percentage of aluminum varies vertically.
 13. The method of claim11, wherein said forming on the epitaxial layer a strongly P-dopedregion of said III-N material comprising a percentage of aluminumcomprises growing an AlGaN layer on a Ga face of the GaN Epitaxiallayer; wherein the percentage of aluminum of the AlGaN layer decreasesfrom bottom to top.
 14. The method of claim 12, wherein said underlyingGaN layer is a strongly P-doped region formed in said epitaxial layerabove the lightly N-doped drift region.
 15. The method of claim 13,wherein the percentage of aluminum decreases from 20 to 0%.
 16. Themethod of claim 11, wherein said forming on the epitaxial layer astrongly P-doped region of said III-N material comprising a percentageof aluminum comprises growing an AlGaN layer on an N face of the GaNEpitaxial layer; wherein the percentage of aluminum of the AlGaN layerincreases from bottom to top.
 17. The method of claim 16, wherein thepercentage of aluminum increases from 0 to 20%.
 18. The method of claim10, wherein said forming on the epitaxial layer a strongly P-dopedregion of said III-N material comprising a percentage of aluminumcomprises growing said region comprising a percentage of aluminum on aP-doped base layer formed in the epitaxial layer on top of the driftregion. 19-20. (canceled)